The filtering had three primary rationales, according to the report: politics and power, security concerns and social norms.

The report said: "In a growing number of states around the world, internet filtering has huge implications for how connected citizens will be to the events unfolding around them, to their own cultures, and to other cultures and shared knowledge around the world."

The report said net censorship was spreading across the globe

Jonathan Zittrain, Professor of Internet Governance and Regulation at Oxford University, said the organisation was also looking at the tools people used to circumvent filtering.

"It's hard to quantify how many people are doing this. As we go forward each year we want to see if some of these circumvention technologies become more like appliances and you just plug them in and they work," he added.

"Few states restrict their activities to one type of content," said Rafal Rohozinski, Research Fellow of the Cambridge Security Programme.

He added: "Once filtering is begun, it is applied to a broad range of content and can be used for expanding government control of cyberspace. It has become a strategic forum of competition between states, as well as between citizens and states."

Mr Palfrey said the report was an attempt to shine a spotlight on filtering to make it more transparent.

"What's regrettable about net filtering is that almost always this is happening in the shadows. There's no place you can get an answer as a citizen from your state about how they are filtering and what is being filtered."